By Katherine Derbyshire

Laser annealing of ultrashallow junctions is one of those technologies that’s been talked about for years, but has been slow to actually reach manufacturing floors. As with many such technologies, though, the semiconductor industry’s needs seem to be catching up with laser annealing’s capabilities. Scott Zafiropoulo, VP of Marketing at Ultratech, said that 2010 was a strong year for laser annealing, with mass adoption and multiple system orders for production at the 45 nm and smaller nodes. He expects the trend to continue in 2011 — Ultratech recently told Wall Street analysts it expects laser annealing system sales to double this year.

Laser annealing adoption is driven by the basic challenge of ultrashallow junction fabrication: the need to achieve a high temperature for dopant activation, while minimizing diffusion so that dopants stay where the ion implanter put them. Rapid thermal processing (RTP), flash lamp annealing, and laser annealing all strive to achieve this combination. As junctions get shallower and dopant concentrations increase, the acceptable dwell time at the activation temperature is dropping into the microsecond range.

Flash lamps, the other leading alternative for millisecond anneals, heat the entire wafer. Due to the wafer’s thermal mass, cool down time is longer and diffusion can occur. Laser annealing, in contrast, scans a laser across the wafer, heating only a small area at once. With temperatures just below the melting point of silicon and dwell times of a few hundred microseconds, the laser annealing process is effectively diffusionless.

As Swami Srinivasan, global product manager for anneals and epi at Applied Materials, pointed out, diffusionless activation isn’t quite what fabs are looking for. The activation anneal also helps to remove defects left by the implant step. Some dopant diffusion is needed to ensure uniform resistivity through the full junction depth, and to control the overlap between the junctions and the gate. Most source/drain anneals, Srinivasan said, combine a spike anneal for defect annealing and diffusion with a high temperature laser annealing step to ensure full activation.

The emergence of millisecond annealing (MSA) as a production technology has been marked by increasing attention to productivity and yield. Such rapid heating and cooling can induce stress dislocations and wafer warping. The stress can disrupt carefully engineered strained channel structures. Lithographers, already struggling to meet the strict overlay requirements of double patterning schemes, do not appreciate additional wafer warpage. Stress, like thermal budget, must now be controlled throughout the transistor process.

Somewhat counterintuitively, laser annealing turns out to reduce wafer stress compared to other MSA approaches. The reason, according to Jeff Hebb, Ultratech’s VP of Laser Product Marketing, is the extremely short dwell time, as low as 200 microseconds in the company’s recently announced model LSA101. Dislocations form when point defects diffuse to a nucleation site. When the dwell time is extremely short, the temperature difference that provides the driving force for diffusion is simply gone before dislocations can form. Flash annealing, in contrast, maintains a temperature gradient for a longer time.

Perhaps the most important yield issue for MSA, however, is pattern dependence. The wafers being subjected to the process have patterned structures, including dielectric layers and a variety of ion implants. These change the optical reflectance of the film, and therefore the amount of optical radiation absorbed and the rate of heating. Some integration schemes use absorber layers to “smooth out” the surface optical characteristics. However, each absorber layer requires, at a minimum, clean, deposition, removal, and post-removal clean steps, which cumulatively represent a substantial increase in process cost and yield risk.

According to Hebb, use of a grazing incidence laser source largely eliminates pattern dependence without the need for an absorber layer. In Ultratech’s tool, p-polarized light from a CO₂ laser — with a wavelength of 10.6 microns, much greater than the device feature size — strikes the wafer at Brewster’s angle. Under these conditions, reflectance becomes zero regardless of the composition of the surface, and pattern dependence is minimal. (For further discussion of pattern dependence issues, see T. Miyashita et. al., IEDM-09.)

Another mark of the increasing maturity of laser annealing is that process engineers are considering applications outside of junction activation. Nickel silicide contacts face a similar need for diffusionless processing: Srinivasan explained that nickel tends to diffuse along defects, including the silicide-silicon interface. These so-called “piping” defects contribute to junction leakage. (The impact of annealing on piping defects is discussed in more detail in Y. He et. al., RTP-2010.)

For junction activation, the chuck temperature is near 400°C, with a process temperature around 1350°C, just below the melting temperature of silicon. Silicide formation, in contrast, is typically a two-step process. First, a low temperature anneal at around 400ºC diffuses nickel deposited on the surface into the silicon. After any excess nickel is etched away, a high temperature anneal at around 850ºC drives NiSi formation. This step is very temperature-sensitive: the paper by Y. He cited above found that resistance increases sharply for process temperatures above 900ºC, due to increasing NiSi₂ formation. Silicidation also requires a lower chuck temperature, between 100º and 200ºC, to minimize thermal budget.

Update: A previous version of this article incorrectly referred to Ultratech’s new laser annealing tool as the model LSA 100A. The LSA 101 is the newest model of this tool.

Tags: Applied Materials, Ultratech

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